The availability of new modulation formats combined with the appearance of a flexible Dense Wavelength Division Multiplexing (DWDM) grid has led to new opportunities and challenges for improving the operation of optical transport networks.
More specifically, DWDM planning is a critical task for network deployments, and concerns the assignment of resources to traffic demands such that some criteria (e.g., total cost, spectrum consumption) is respected, improved or optimized. This task can be usually divided in two flavors: offline and online planning. The latter consists of assigning resources (routing path and spectral window) to new traffic requests as they arrive in the network. With only the knowledge of the current traffic demand and the already existing traffic, planning decisions are taken with a local scope, which can lead to scalability issues when the network grows. In view of this, attempts have been made, also in online applications, to plan ahead and provision the network infrastructure based on traffic forecasts built upon past history and future traffic expectations.
In traditional fixed-grid networks, this planning process targets lowering the provisioning costs by balancing the load over the available network links, deciding where/when to light up new fibers and efficiently grooming client signals over the existing capacity. In this respect, the above mentioned availability of new modulation formats combined with a finer granularity flexible DWDM grid has greatly expanded the set of possible transmission formats which are available for transporting traffic. In particular, there now exists the concept of a Media Channel (MCh), where multiple optical carriers (which would previously be individual channels) between the same node-pair can be spectrally and logically aggregated into a single network entity and co-routed through the network. The optical path carrying the Media Channel (MCh) is defined by the set of links it uses to connect its end-nodes, and the format of the Media Channel (MCh) can be specified by, for example: the number and type of carriers it contains (i.e., which modulation format is used), the spectral spacing between those carriers and the guard-bands between the MCh edge and the carrier closest to that edge. A corresponding MCh 100 through path, defined by the network nodes A-B-C, and having three optical carriers 110 separated by carrier spectral spacing 120 and including guard-bands 130 is shown in FIG. 1.
In other words, the availability of new modulation formats combined with the appearance of a flexible Dense Wavelength Division Multiplexing (DWDM) grid has led to the emergence of the Media Channel (MCh) concept, wherein multiple individual carriers between the same end-nodes are logically aggregated, for example in a single layer-0 container, and can be viewed by a respective control plane as a unique entity. A significant advantage of the MCh scheme is that Media Channels (MChs) enable a more efficient spectrum utilization as the multiple carriers inside a MCh effectively share the same guard-band; see further discussion in “On the impact of optimized guard-band assignment for superchannels in flexible-grid optical networks” by A. Eira, J. Pedro, J. Pires, in Proc. Optical Fiber Communication Conference (OFC), paper OTu2A.5, March 2013. In this respect, in order to ensure that optical performance thresholds are not violated, each MCh includes on both its edges a guard-band of unused spectrum to reduce interference from adjacent MChs. It follows that it is spectrally efficient to have as few MCh containers per node-pair as possible, in order to reduce the amount of spectrum that must be provisioned for guard-bands.
Seizing the spectral benefits of a flexible-grid and MChs requires a planning method that is aware of the importance of efficient guard-band utilization, for example by minimizing the amount of spectral containers between node-pairs. In an offline planning application where all traffic is known, optimization methods can be designed to plan all demands by simultaneously minimizing the set of required MCh containers. By contrast, in online applications, the network management system/Software-defined Networking (SDN) controller only has information about the current service to provision and the existing network state (deployed MChs). Hence, without any other knowledge or prediction capability, service provisioning must use whatever MCh capacity is available or else create new containers. In the long-run, operating the network in this fashion may create inefficiencies in the use of available spectrum by unnecessarily creating multiple MChs between the same node-pairs and thus wasting the spectrum used for guard-bands in each of them. FIG. 2 shows an example of how the spectrum on a given network link may look like when using single-carrier channels (A), when using MChs without optimizing the number of guard-bands (B), and how the spectrum should ideally look like in order to use the least spectrum resources possible (C). This introduces in the planning process a new degree of “optical grooming”, where it is desirable to direct new optical carriers towards existing MCh containers in order to improve the overall spectrum utilization; see further discussion in “Dynamic traffic grooming in sliceable bandwidth-variable transponder-enabled elastic optical networks” by Zhang, Y. Ji, M. Song, Y. Zhao, X. Yu, J. Zhang, B. Mukherjee, Journal of Lightwave Technology, vol. 33, no. 1, pp. 183-191, January 2015. Thus, network planning methods including routing and spectrum assignment must account for the reach/capacity trade-off inherent to the availability of multiple modulation formats, as well as the spectral dividend yielded by reducing the number of logical channels between end-nodes.
Conventionally, routing and spectrum assignment engines are embedded in network management systems/Software-defined Networking (SDN) controllers for online operation, and employ a simple resource management strategy by: simply checking for existing capacity at the Media Channel (MCh) layer, and deploying new carriers in existing Media Channels (MChs) if possible, or otherwise creating new Media Channel (MCh) containers; wasting the spectrum required for provisioning new guard-bands. Moreover, the creation of these containers is usually associated to a specific transponder card or follows a fixed rule (i.e., always the same size of container), which further adds to the spectral inefficiency of these methods.
It follows that conventional online planning methods exploiting MChs do not envision a global spectrum reservation policy to reduce the inefficiencies generated by guard-bands. In some cases, spectrum is reserved for the transponder module itself, based on the number of subcarriers it supports; see “Dynamic traffic grooming in sliceable bandwidth-variable transponder-enabled elastic optical networks” by Zhang, Y. Ji, M. Song, Y. Zhao, X. Yu, J. Zhang, B. Mukherjee, Journal of Lightwave Technology, vol. 33, no. 1, pp. 183-191, January 2015. This assumes that a MCh is directly associated to a specific transponder module/card, and thus Emits the flexibility of the MCh itself. Furthermore, it does not incorporate any forecast/historical data and so it is unable to properly coordinate the distribution of the available spectral resources among the various MChs.
In “Blocking probability and fairness in two-rate elastic optical networks” by X. Wang, J. Kim, S. Yan, M. Razo, M. Tacca, A. Fumagalli, in Proc. International Conference on Transparent Optical Networks (ICTON), paper Th.B3.2, July 2014, a method is proposed to partition the spectrum among various channels' rates (modulation formats) in order to reduce the blocking probability in dynamic scenarios. This approach pertains only to the division among different channel widths to reduce fragmentation, but does not consider the distribution of spectrum within each rate to each particular node-pair, and hence cannot have a global optimized view of how to maximize throughput in the network. The solution in “Routing and spectrum allocation method for immediate reservation and advance reservation requests in elastic optical networks” by S. Sugihara, Y. Hirota, S. Fujii, H. Tode, T. Watanabe, in Proc. Photonics in Switching (PS), pp. 178-180, September 2015, proposes a “soft-reservation” scheme where spectrum is reserved in bands for immediate and advanced reservation, but separating the bands means that the benefit of optical grooming is not duly exploited. Nor does the proposal consider the relative weight of the overall traffic distribution, or network-wide contention for the links used in each path to distribute the spectrum among each node-pair.